Filtration process and apparatus

ABSTRACT

It is possible to filter a suspension or emulsion in a filter bed of particulate filter medium by allowing the suspension to flow upwards through the filter bed simultaneously as the medium flows downwards through a filtering tank wherein filtration takes place. Filtration thus takes place during counterflow between suspension and filter medium. The supply of suspension takes place in the lower part of the filter bed and the filtered liquid phase is taken away from a zone above the filter bed. The polluted filter medium close to the bottom of the filter tank is taken out for conveyance to a washing apparatus above the filter bed and after washing is returned to the upper side of the filter bed. For providing a filter which has a considerably lower construction height for a given and unchanged volume of the filter medium, the washing apparatus is lowered so that its lower part is at a level below the upper surface of the filter bed.

FIELD OF THE INVENTION

The present invention relates to a method of filtering a suspension oremulsion, and also relates to a filtering apparatus for carrying out themethod.

BACKGROUND OF THE INVENTION

It may be read from the Swedish patent specification 396 552 thatfiltering a suspension or emulsion in a filter bed comprisingparticulate filter material may be carried out by allowing thesuspension to flow upwards through the filter bed, simultaneously as thefilter medium flows downwards through a filtering tank in which thefiltration takes place. Filtering thus takes place during counter flowbetween suspension and filter medium. The suspension is supplied at alower part of the filter bed and the filtered liquid phase is taken awayfrom a zone above the filter bed. The polluted filter medium close tothe bottom of the filter tank is taken out and conveyed to a washingapparatus above the filter bed, and after washing it is returned to theupper side of the filter bed. By continuous supply of suspension to thefilter bed, continuous extraction of filtered liquid phase as well ascontinuous conveyance of polluted filter medium to the washing apparatusand washed filter medium back to the filter bed results in that thefiltering apparatus operates completely continuously, and does notnormally need to be taken out of operation.

Such filtering apparatus may reach considerable dimensions in order toreceive large suspension flows. In large installations there may be aplurality of filtering units, which are placed side by side and work inparallel, this being more practical or more suited to the availablespace. A method of arranging a large installation of this kind isillustrated in FIG. 3 of the previously mentioned patent specification396 552. A normal circular shape of the filtering units has beendeparted from here, and a hexagonal shape has been adopted to achievecompaction of the units. Since conveyance of the filter medium to thewashing apparatus takes place through the filter bed, and the washingapparatus is also within the outer shell of the unit, there is nothingto prevent such compaction of such filtering units.

A number of factors affect the selection of the filter apparatus size,such as type of suspension or emulsion, desired purification degree,type of filter medium and, as previously mentioned, the magnitude of thesuspension flow that is to be filtered. These factors determine the areaand height of the filter bed body or bodies. The filtering apparatus isthen designed from these dimensions, e.g. as is illustrated in FIG. 1 ofthe mentioned patent specification 396 552.

Although this figure is only a principle sketch, the proportions of thefigure do not differ from reality to any notable degree. Accordingly,the observation may be made that a considerable portion of the height ofthe filtering apparatus has been used to make room for the pollutedfilter medium washing apparatus. In the case where the available heightis limited in the space where the filter units are to be placed, theheight of these tanks will cause some limitations in the design.

SUMMARY OF THE INVENTION

The present invention has come about to provide a filter, which has aconsiderably lower construction height for a given and unchanged volumeof the filter medium. In accordance with the invention this has beenarranged by sinking the lower part of the washing apparatus to a levelunder the upper surface of the filter bed, but with retention of acharacteristic method of carrying out washing. This method of washinginvolves the provision of an upward current of washing liquid,constituting the ready-filtered liquid phase, in a washing path wherethe polluted filter medium is caused to fall downwards against thewashing liquid flow. Since the washed filter medium comes in at a levelwhich is too low in relation to the filter bed, due to sinking the levelof the washing apparatus, extra conveying equipment for the filtermedium up to the upper side of the filter bed will be required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the form of a preferred embodimentand with reference to the accompanying drawing figure. The latterillustrates a filtering apparatus in vertical section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the illustrated embodiment, the particulate filter medium 1 isenclosed in a container with walls 2 and a bottom 3 having a conical orpyramidical shape with the apex downwards. The base of the cone orpyramid should suitably conform to the shape of the container as definedby the walls 2.

Sand is well suited as particulate filter medium, although othermaterials, such as plastics may be used, or mixtures of severalmaterials. After trials, one skilled in the art will find the mostsuitable material and particle size in each individual case, dependingon what is to be filtered and how effectivly filtration shall takeplace.

The suspension or emulsion which is to be filtered is taken, asillustrated by the arrow A into the container via the inlet 4, e.g.through the bottom of the container. The inflow of the suspension in thefilter bed takes place at the lower part of the bed via a plurality ofpipes 5 with pipe openings 6. Above each of the pipe openings there is ashield 7 for preventing direct contact of the filter medium with thepipe openings and for exposing a larger surface of the filter medium tothe incoming suspension. Alternatively, the suspension can be taken infrom above through unillustrated pipes and under the shields 7. The riskof clogging the filter due to heavy instantaneous pollution of thefilter medium adjacent the pipe openings is thus reduced. By the shieldshaving a V-shape, and by placing the pipe openings under shields, thereis achieved the least possible obstruction to the downward passage ofthe filter medium. Placing the inlet pipe openings 6 in the lower partof the filter bed results in the advantage that the most polluted partof the filter material, i.e. that which goes past the shields and thepipe openings, continues downwards and is not utilized further forfiltration. Blockage at the inlets is thus prevented, and new filtermedium is continuously exposed to the incoming suspension. The latterflows upwards in a direction counter to that of the filter mediumthrough a zone of the filter bed towards an increasingly cleaner medium.The filtered liquid phase, obtained as a result of the upward flow ofthe suspension through the filter bed zone, is maintained as a zone 8 offiltered liquid phase above the filter bed, and in the illustratedembodiment example, its level is determined by the filtered liquid phasebeing taken out from the filtering apparatus via an overflow 9 to anoutlet (arrow B).

The opening of some suitable elevating means 10 is situated in theconically or pyramidically shaped portion 3 of the container down whichflows the filter medium polluted during filtration. As will be seen fromthe illustrated embodiment example, the elevating means extendscentrally through the apparatus and can be such as a so-called mammothpump. This type of pump uses air as conveying medium, and the air issupplied from above and down a conduit 11 extending along a conveyerpipe 12 included in the elevating means. Air is supplied through theconveyer pipe via unillustrated holes at the lower end of the conduit.The filter medium is conveyed (arrow C) by the air through the pipe 12to a washing apparatus 13 situated in the filter bed.

The filter medium to be washed comes out from the upper part of theconveyer pipe 12, changes direction (arrow D), and runs downwards in awashing path formed between the elevating means 10 and an outer shell 14of the washing apparatus. The outer shell 14 is suitably concentricallyformed around the elevating means 10. Due to its being polluted, thefilter medium may contain cohesive aggregats. These are broken down whenthey meet obstacles 15 in the washing path. The obstacles 15 are formedsuch that they force the filter medium to change direction somewhat, atthe same time as its fall rate changes.

All the time it is in the washing path the filter medium meets a washingmedium moving in a counter direction. The filtered liquid phase in thezone 8 above the filter bed is used as washing liquid. This zone 8 is incommunication with the lower part of the washing path via a space 16between the washing apparatus and an outer casing 17 forming a shieldagainst the bed. The washing liquid is thus taken in at the lower partof the washing path, as illustrated by the arrow E in the embodimentexample.

The washing liquid, or reject, polluted by washing, is taken from thewashing path, as illustrated by the arrow F and is taken out via anoverflow 18 and discharged from the apparatus, e.g. as illustrated bythe arrow G. This overflow 18 should be adjustable in height andpossibly in its width as well, to enable regulation of the washingliquid amount through the washing path. If the overflow is lowered, thewashing liquid amount increases. This amount can also be affected by theclear solution level in the zone 8 being regulated by the overflow 9. Itis the difference in level between the clear solution level in the zone8 and the washing water level in the zone 8a above the washing apparatusthat determines the washing liquid amount.

The filter medium, that has been washed during its passage along thewashing path, is taken down to a lowest point with the aid of an obliquebottom 19 under the washing apparatus. Immediately above this lowestpoint there is the opening of a further conveying means 20 for thefilter medium. In the embodiment example illustrated here, this alsocomprises a mammoth pump, e.g. with air as the conveying medium. Thepipe of the conveying means 20 extends up to a level at a height abovethe upper side of the filter bed such that, with the aid of adistribution plate 21 provided with a roof it is possible to distributethe filter medium over the upper side of the filter bed.

Within the framework of given premises, i.e. the method and apparatusfor carrying out filter medium washing, there has been achieved with theaid of the present invention a variant of the filtering unit, which hasa considerably reduced construction height. In the described, andperhaps the most advantageous embodiment, the washing apparatus is stillwithin the same configuration of the filtering unit since it has beensunk into the space for the filter medium itself. The filtering unit isconsequently given the least possible space, not only in height but alsolaterally. The possibility still remains of compactly placing aplurality of filtering units side by side.

We claim:
 1. Method for filtering a suspension or emulsion, said methodcomprising:supplying the suspension or emulsion to a zone of particulatefilter medium forming a filter bed, flowing said suspension or emulsionupwards through the filter bed during filtration and departing from thefilter bed as a filtered liquid phase, flowing the particulate filtermedium during filtration in a direction downwards through the zone in acounter flow to the suspension or emulsion past a point of entry of thesuspension or emulsion for subsequent conveyance separate from thefilter bed to a substantially vertical washing path disposed within saidfilter bed for washing the filter medium polluted by filtration, washingof the filter medium taking place during counter flow with respect to awashing liquid along the washing path after which the washed filtermedium is returned to an upper side of the zone, taking polluted filtermedium from a low point in the filter bed via the substantially verticalwashing path and leaving said washing path at a level below that of afilter bed surface, at which level the washing liquid is supplied tosaid washing path, the washing liquid thus flowing in at an inlet end ofthe washing path and departing in a polluted state at a level above theinlet end to the path, and conveying the cleaned filter material from apoint below the outlet of said washing path up to a level above saidfilter bed surface, where a zone of filtered liquid phase is maintained.2. Method as claimed in claim 1, wherein the filter medium is subjectedto changes in flow rate during passage along the washing path.
 3. Methodas claimed in claim 1, wherein a part of the filtered liquid phase abovethe filter bed is used as washing liquid.
 4. Method as claimed in claim1, wherein the polluted washing liquid is taken from the washing path ata level below the level of the filtered liquid phase.
 5. Filteringapparatus for filtering a suspension or emulsion, said filteringapparatus comprising:a filter tank, a zone of particulate filter mediumforming a filter bed extending vertically in said filter tank; an inletto said filter tank for the suspension or emulsion which is to befiltered, said inlet being situated in a lower part of the filter bedzone, the suspension flowing in a direction upwards through the filterbed during filtration; a space in said filter tank situated under theinlet and in free communication with the filter bed zone, the filtermedium continuously being supplied to said space by the medium flowingin a direction downwards through the filter bed zone; conveying meansopening out into the space for elevating filtration-polluted filtermedium which has been supplied to said space and for conveying thefiltration-polluted filter medium to a washing apparatus in said filtertank separate from the filter bed; said washing apparatus being passedthrough by the filter medium and a washing liquid for washing the filtermedium in counter-flow, the washed filter medium being taken from thewashing apparatus and returned to an upper side of the filter bed zone;and an outlet for washing liquid polluted by said washing located at anupper part of the washing apparatus, the washing apparatus including avertical cylinder, a lower end of said washing apparatus being situatedat a level lower than an upper surface of the filter bed, and at a givendistance above an intake of the conveying means for the polluted filtermedium, a reception space being located under said vertical cylinder forwashed filter medium, a second conveying means having an intake and anoutlet, the intake of the second conveying means being located in saidspace, the outlet of said second conveying means being located above theupper surface of the filter bed, an inlet to said vertical cylinder forthe washing liquid being located at a lower end of said verticalcylinder, and an outlet of the washing liquid from the washing apparatusbeing located above an upper end of said vertical cylinder.
 6. Filteringapparatus as claimed in claim 5, wherein the outlet of the washingliquid is lower than a level of the filtered liquid phase located abovethe filter bed.
 7. Filtering apparatus as claimed in claim 6, wherein aheight difference between said outlet and said level is regulatable. 8.Filtering apparatus as claimed in claim 6, wherein the washing apparatusis situated inside the filter bed and is surrounded by a chamber with anopening extending upwards towards the level of the filtered liquidphase, the lower end of said vertical cylinder being in communicationwith a bottom portion of said chamber, said bottom portion defining saidspace where said second conveying means has said intake.
 9. Filteringapparatus as claimed in claim 8, wherein above the vertical cylinder isa collection chamber for the polluted washing liquid.